1. Field of the Invention
The present invention relates to micro-switching devices manufactured by means of MEMS technology.
2. Description of the Related Art
In the field of radio communications equipment such as mobile telephones, there has been an increasing demand for smaller radio frequency circuitry in order to meet e.g. increase in the number of parts which must be incorporated for higher performance. In response to such a demand, size reduction efforts are being made for a variety of parts necessary for constituting the circuitry, by using MEMS (micro-electromechanical systems) technology.
MEMS switches are examples of such parts. MEMS switches are switching devices in which each portion is formed by MEMS technology to have minute details, including e.g. at least one pair of contacts which opens and closes mechanically thereby providing a switching action, and a drive mechanism which works as an actuator for the mechanical open-close operations of the contact pair. In switching operations particularly for high-frequency signals in the Giga Hertz range, MEMS switches provide higher isolation when the switch is open and lower insertion loss when the switch is closed, than other switching devices provided by e.g. PIN diode and MESFET because of the mechanical separation achieved by the contact pair and smaller parasitic capacity as a benefit of mechanical switch. MEMS switches are disclosed in e.g. JP-A-2004-1186, JP-A-2004-311394, JP-A-2005-293918, and JP-A-2005-528751.
FIG. 21 through FIG. 25 show a conventional micro-switching device or a micro-switching device X4. FIG. 21 is a plan view of the micro-switching device X4, and FIG. 22 is a partial plan view of the micro-switching device X4. FIG. 23 through FIG. 25 are sectional views taken along lines XXIII-XXIII, XXIV-XXIV and XXV-XXV respectively in FIG. 21.
The micro-switching device X4 includes a base substrate S4, a fixing member 41, a movable part 42, a contact electrode 43, a pair of contact electrodes 44A, 44B (not illustrated in FIG. 22), a driver electrode 45, and a driver electrode 46 (not illustrated in FIG. 22).
As shown in FIG. 23 through FIG. 25, the fixing member 41 is bonded to the base substrate S4 via the boundary layer 47. The fixing member 41 and the base substrate S4 are formed of monocrystalline silicon whereas the boundary layer 47 is formed of silicon dioxide.
As shown in FIG. 22 and FIG. 25 for example, the movable part 42 has a stationary end 42a fixed to the fixing member 41, as well as a free end 42b. The movable part extends along the base substrate S4, and is surrounded by the fixing member 41 via a slit 48. The movable part 42 is formed of monocrystalline silicon.
As shown clearly in FIG. 22, the contact electrode 43 is near the free end 42b of the movable part 42. As shown in FIG. 23 and FIG. 25, each of the contact electrodes 44A, 44B is formed on the fixing member 41 and has a portion facing the contact electrode 43. Also, the contact electrodes 44A, 44B are connected with a predetermined circuit selected as an object of switching operation, via predetermined wiring (not illustrated). The contact electrodes 43, 44A, 44B are formed of a predetermined electrically conductive material.
As shown clearly in FIG. 22, the driver electrode 45 extends on the movable part 42 and over to the fixing member 41. As shown clearly in FIG. 24, the driver electrode 46 has its ends bonded to the fixing member 41 so as to bridge over the driver electrode 45. Also, the driver electrode 46 is grounded via predetermined wiring (not illustrated). The driver electrodes 45, 46 are formed of a predetermined electrically conductive material. The driver electrodes 45, 46 as described above serve as a drive mechanism in the micro-switching device X4, and has a driving force generation region R′ on the movable part 42 as shown in FIG. 22. As shown clearly in FIG. 24, the driving force generation region R′ is a region facing the driver electrode 46, in the driver electrode 45.
In the micro-switching device X4 arranged as described above, electrostatic attraction is generated between the driver electrodes 45, 46 when an electric potential is applied to the driver electrode 45. With the applied electric potential being sufficiently high, the movable part 42, which extends along the base substrate S4, is elastically deformed until the contact electrode 43 makes contact with the contact electrodes 44A, 44B, and thus a closed state of the micro-switching device X4 is achieved. In the closed state, the pair of contact electrodes 44A, 44B are electrically connected with each other by the contact electrode 43, to allow an electric current to pass through the contact electrodes 44A, 44B. In this way, it is possible to achieve an ON state of e.g. a high-frequency signal.
On the other hand, with the micro-switching device X4 assuming the closed state, if the application of the electric potential is removed from the driver electrode 45 whereby the electrostatic attraction acting between the driver electrodes 45, 46 is cancelled, the movable part 42 returns to its natural state, causing the contact electrode 43 to come off the contact electrodes 44A, 44B. In this way, an open state of the micro-switching device X4 as shown in FIG. 23 and FIG. 25 is achieved. In the open state, the pair of contact electrodes 44A, 44B are electrically separated from each other, preventing an electric current from passing through the contact electrodes 44A, 44B. In this way, it is possible to achieve an OFF state of e.g. a high-frequency signal.
In order to achieve the above-described closed state, the electric potential, i.e. driving voltage, to be applied to the driver electrode 45 in the micro-switching device X4 is often designed to be large, for the following reasons:
When the micro-switching device X4 is manufactured, the contact electrode 43 is formed by means of thin-film formation technology, on the movable part 42, or more accurately, at a predetermined place of formation where the movable part is to be formed on a material substrate. Specifically, the contact electrode 43 is formed by first forming a film of a predetermined electrically conductive material by spattering, vapor deposition, etc., on a predetermined surface, and then by patterning the film. The contact electrode 43 formed by thin-film formation technology usually has a certain amount of internal stress. As shown exaggeratingly in FIG. 26(a) and in FIG. 26(b) for example, the internal stress deforms a portion of the movable part 42 which is supposed to make contact with the contact electrode 43, as well as the region surrounding the portion, together with the contact electrode 43. Once such a deformation occurs, the distance between the two contact electrodes 43, 44A is often no longer equal to the distance between the contact electrodes 43, 44B, in a non-activated state i.e. the open state of the switch.
FIG. 27 shows an example process where the micro-switching device X4 changes its state from the open state to the closed state. FIG. 27(a) through FIG. 27(c) each include a partial enlarged section of the open/close point between the contact electrode 43 and the contact electrode 44A and a surrounding region, as well as a partial enlarged section of the open/close point between the contact electrode 43 and the contact electrode 44B and a surrounding region.
FIG. 27(a) shows an open state where the distance between the contact electrodes 43, 44A is smaller than the distance between the contact electrodes 43, 44B. If a voltage applied between the driver electrodes 45, 46 is gradually increased from 0 V, the electrostatic attraction between the driver electrodes 45, 46 also increases gradually, and because of this electrostatic attraction, the movable part 42 which extends along the base substrate S4 makes partial elastic deformation, and at a certain voltage V11, the gap between the contact electrodes 43, 44A is closed as shown in FIG. 27(b). During such a process (the first process) from the open state shown in FIG. 27(a) through an intermediate state shown in FIG. 27(b), bending deformation occurs mainly in a portion of the movable part 42 ranging from a region corresponding to the driving force generation region R′ shown in FIG. 22 to the stationary end 42a. The first process can also be described as follows: Namely, a force acts on the movable part 42 through a mechanism where the stationary end 42a of the movable part 42 functions as a fulcrum point or a fixed axis, with a working point of the force being the center of gravity C′ of a portion (driving force generation region R′) indicated in FIG. 22 as a region in the driver electrode 45 facing the driver electrode 46.
After the gap between the contact electrodes 43, 44A is closed as shown in FIG. 27(b), the voltage applied between the driver electrodes 45, 46 is increased further, to further increase the electrostatic attraction between the driver electrodes 45, 46. Then, at a certain voltage V12 (>V11), the gap between the contact electrodes 43, 44B is closed as shown in FIG. 27(c). In such a process (the second process) from the intermediate state shown in FIG. 27(b) through the closed state shown in FIG. 27(c), torsional deformation occurs mainly in the portion of the movable part 42 ranging from the region corresponding to the driving force generation region R′ to the stationary end 42a. The second process can be described as follows: Namely, a force acts on movable part 42 through a mechanism shown in FIG. 22, where a virtual line F′ which passes through the stationary end 42a of the movable part 42 and the point of contact provided by the contact electrodes 43, 44A represents a fixed axis or an axis of rotation, with a working point of the force being the center of gravity C′ of the driving force generation region R′.
On the other hand, when the closed state is achieved in a micro-switching device X4 where the distance between the contact electrodes 43, 44A is larger than the distance between the contact electrodes 43, 44B in the open state, the gap between the contact electrodes 43, 44B is closed first and thereafter, the gap between the contact electrodes 43, 44A is closed.
In order to achieve a closed state in the micro-switching device X4, two processes are required for example as described above, i.e. the first process which is a process from the open state to the intermediate state in FIG. 27(b), and the second process which is a process from the intermediate state to the closed state shown in FIG. 27(c). The first process and the second process differ from each other in the mode of deformation of the movable part 42. In the deformation mode of the first process, the stationary end 42a of the movable part 42 acts as a fulcrum point or a fixed axis, and the distance between the fixed axis and the center of gravity C′ of the driving force generation region R′ (working point) is relatively long. For this reason, the first process requires a relatively small driving voltage V11 or electrostatic attraction for an amount of momentum to be generated in e.g. the center of gravity C′ in order to achieve a required level of deformation in the movable part 42. On the contrary, in the deformation mode of the second process, the virtual line F′ which passes through the stationary end 42a of the movable part 42 and the point of contact provided by the contact electrodes 43, 44A represents a fixed axis or an axis of rotation, and the distance between the axis (virtual line F′) and the center of gravity C′ of the driving force generation region R′ (working point) is substantially short. For this reason, in the deformation mode of the second process, a substantially large driving voltage V12 must be applied between the driver electrodes 45, 46 whereby a substantially large amount of electrostatic attraction must be generated between the driver electrodes 45, 46 in order to generate a sufficient amount of momentum to deform the movable part 42 thereby closing the gap between the contact electrodes 43, 44B.
As has been described, in the conventional micro-switching device X4, the distance between the contact electrodes 43, 44A often differs from the distance between the contact electrodes 43, 44B, and in such a case, the distance between the virtual line F′ (fixed axis) and the center of gravity C′ (working point) in the driving force generation region R′ in the second process is substantially short. Therefore, the micro-switching device X4 often requires a large voltage (driving voltage) in order to achieve the closed state where both of the contact electrodes 44A, 44B make contact with the contact electrode 43.